Over the past several years, quantitative in vitro studies of kinetics and inhibitors have firmly established the general characteristics of sugar, amino acid, and Na ion transport in the intestine and other epithelia, e.g., energy dependence, competition, saturation, ionic dependence, and sensitivity to specific inhibitors. However, if we are to fully understand these transport processes, the basic cellular and molecular mechanisms underlying these characteristics must be explained. This proposal focuses on the in situ molecular properties of the uphill entry processes of sugars and amino acids and location of Na ion transport sites. Specific efforts will be directed to determination of the kinds of protein reactive reagents that combine irreversibly with microvillous membranes to produce inhibition of either sugar or amino acid entry, and to the identification of the functional groups with which these reagents react. In addition H3-ouabain radioautography will be used to localize Na ion transport sites in the intestine and other epithelia.